The present invention relates generally to flowable, setting-type gypsum compositions. More particularly, the invention relates to a flowable, ready-mixed composition comprising calcined gypsum and a set preventer, wherein the composition sets when mixed with an activator for the reaction of calcined gypsum with water to form set gypsum.
Set gypsum (calcium sulfate dihydrate) is a well-known material that is used to make many types of products and that is included commonly in many types of products. Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum (i.e., calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, and optionally other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product.
In many instances, it is desirable to provide a ready-mixed setting-type composition that includes calcined gypsum pre-mixed with water, a set preventer, and other additives as desired. These compositions are generally packaged in a container such as a pail, can, bucket, carton, and the like so that little or no additional water is required to be added to the gypsum pre-mix for use, for example, at the job site. Ready-mixed compositions are designed to prevent setting until the user adds an activator for the reaction of calcined gypsum with water to form set gypsum.
Ready-mixed compositions are well known for use as joint compounds, commonly used to join abutting wallboard or to patch wallboard. U.S. Pat. No. 5,746,822, for example, describes ready-mixed gypsum-based compositions that include non-calcium bearing phosphates as set retarders. According to the ""822 patent, tetra sodium pyrophosphate provides the most long-lasting set retardation effect of the non-calcium bearing phosphates and is used in conjunction with citric acid. The ready-mixed compositions of the ""822 patent are not fully satisfactory for many applications other than as joint compounds because they are formulated to be relatively viscous and they have relatively low strength.
In addition, U.S. Pat. No. 5,779,786 discloses a ready-mixed joint compound that includes a retarder in the form of a polymer composition including acrylic acid and acrylamide monomer units. As examples, the ""786 patent discloses a copolymer of acrylic acid and acrylamide or a blend of a homopolymer of acrylic acid and a homopolymer of an acrylamide. The ready-mixed formulations of the ""786 patent are designed to be relatively viscous to avoid bleeding of water and settling of particulate. The ready-mixed compositions of the ""786 patent also do not exhibit sufficient strength for many applications other than joint compound.
There is a continuing need in the art for ready-mixed compositions that are formulated to have lower viscosity and higher strength than ready-mixed joint compounds. For example, in many applications, it would be desirable to have a ready-mixed composition that is flowable before activator is added to the composition, and which remains flowable for some period of time even after activator is added, so that the user has sufficient xe2x80x9cworkablexe2x80x9d time to use the composition for its intended purpose.
Flowable ready-mixed compositions would be desirable for use in a variety of interior and exterior applications, as one of ordinary skill in the art will appreciate. By way of example, flowable ready-mixed compositions would be useful in a number of plaster applications including dental applications, building plaster, machinable plaster, and the like. For example, dental plaster is useful for casting dimensionally accurate reproductions of a portion of an oral cavity or extraoral facial structure as a positive impression formed from a polymer such as alginate that forms a negative impression. In addition, building plaster may be in the form of, for example, veneer plaster (e.g., DIAMOND(copyright) brand interior finish plaster, commercially available from the United States Gyspum Company) for finishing interior wallboards.
Flowable ready-mixed compositions also would have use in forming molds made of plaster that are used in casting pottery figurines and the like using pressure casting or slip casting techniques known in the art. In addition, ready-mixed flowable compositions also would be useful for solid casting applications, such as, for example, statuary and architectural pieces such as moldings, as well as rotational casting applications. For example, rotational casting is utilized to form hollow products, such as, for example, lamp posts and the like. Flowable ready-mixed compositions also would be useful in spray applications, including glass fiber reinforced gypsum compositions that may be used in making decorative architectural pieces.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for ready-mixed, setting-type compositions that have a relatively low viscosity so that they are flowable, and preferably that remain flowable even after activator is added. It will be appreciated also that there is a need in the art for such a ready-mixed composition in which setting is avoided in the absence of activator for a relatively long time such that the composition has a long shelf-life. It will be appreciated further that there is a need in the art for such a ready-mixed composition in which the solid particulate in the composition does not settle and where water does not bleed. The invention provides such a ready-mixed composition. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a composition that sets when mixed with an activator for the reaction of calcined gypsum with water to form set gypsum. The composition comprises water and calcined gypsum in a weight ratio of water to calcined gypsum of from about 0.15 to about 0.5. The composition also comprises a set preventer comprising a polyacrylic acid and/or a salt thereof. The composition has a viscosity of from about 5,000 centipoise to about 48,000 centipoise. The ready-mixed composition of the present invention has a long shelf-life and will set after the mixing of activator therewith. In some embodiments, the ready-mixed composition exhibits sufficient flowability even after mixture with an activator to permit the activated composition to be applied and formed to a desired shape.
The present invention also provides a kit comprising separately packaged components. One of the components is a composition comprising calcined gypsum and a polyacrylic acid and/or a salt thereof, and, optionally, water. Another component comprises an activator which, when mixed with the composition, allows the composition to set.
The invention may best be understood with reference to the following detailed description of the preferred embodiments.